The present invention generally relates to a calibrator comprising a set of interchangeable pieces, and related method of use thereof, for calibrating a computerized tomographic system.
With the advent of modern technology, a great variety of scanning and/or imaging techniques have been devised which go far beyond simple optical manipulation of images in the visible light range. Certain of the advanced imaging techniques involve systems which have separate components, such as radiation or exposure sources for exposing an object to be scanned, and various detection devices for detecting resulting interference or the like to the radiation source output, due to the presence, density, etc. of the object to be scanned. Significantly, the interpretation of the detection data may be accomplished substantially in software/hardware combinations, which have settable operating variables which significantly alter the images formed. In other words, a given image generated with software will depend on selection of software variables as much as hardware detection data. Such variables may be several (or more) in number, and may have complex interrelationships, such that their adjustment is not a simple task. Moreover, their adjustment would necessarily vary from one "set-up" to another, just as it is necessary to adjust a camera for focus and the like when going from composing one image to another.
In the case of computerized tomography (CT), a two dimensional picture is created, representative of a plane of interest in a given object. For example, in industrial CT operations, it may be desirable to scan or test a given workpiece for defects. One example would be to check for cracks in a pump or fluid control valve, such as might be used in a space shuttle or in any number of critical applications where an exceedingly high degree of reliability is involved. Nuclear station cooling and control systems is but one further example of such needs.
In such situations, the requisite spatial and contrast resolution and detectability of a CT system will be influenced by various parameters of the scanning or testing to be done. That is, the "focusing" or "aligning" of the CT system will depend on the anticipated size of defects for which the object or workpiece is being examined, relative to the size, thickness, etc. of the workpiece. Since the penetration of various radiation sources is involved, the absolute density and relative densities of the materials involved is also a factor, both in selection of the physical arrangement of the CT system (e.g., selection of exposure source, position of exposure source versus object to be scanned and detector array), as well as selection of the software operating variables. Therefore, the idea and need for a simple and easy CT calibration technique is recognized herewith as fundamentally different from the idea and use of a "phantom", which relates to an exact prototype of the workpiece to be scanned.
The medical profession has made growing use of a Computerized Axial Tomographic (CAT) system, including use of so-called medical phantoms which "mock-up" typical human body dimensions and densities. Such phantoms are limited in their range of object densities, and normally approximate about 1 gm/cc. The needs related to industrial computerized tomographic systems differ in large measure because of significant differences in the objects to be scanned. For example, the range of densities in industrial workpieces to be scanned can easily cover about 0 gm/cc to about 10 gm/cc. Also, size is a consideration, wherefore industrial workpieces requiring examination can easily involve objects in excess of five feet by six feet in measurement, and weighing over one ton. Normally, the typical medical targets would not have such a range of size and weights.
Some industrial phantoms have been developed to either mock-up a unique or specific industrial device, such as a rocket motor, or have been developed to test system detectability and so-called contrast resolution over relatively narrow ranges of size and density. Examples of such phantoms are the Sieman's Star, the NSF (National Science Foundation) phantom, and certain rocket motor phantoms. Another example generally is the AAPM (medical) phantom. Such phantoms do not typically have uniform geometric features, and do not permit significant variation in the ranges of size and densities which may be provided. For example, the AAPM phantom is a generally plastic structure permitting focus and resolution only in that given density. The NSF phantom provides a series of smaller and smaller plates, but with no significant degree of density variation. Also, the intended purpose of such structures is primarily to be a mock-up for a particular device or structure to be scanned, rather than to aid in the general calibration of the CT system (whether medical or industrial in character).